Regulation of tissue growth in feedlot steers through endogenous and repartitioning mechanisms

Typescript (photocopy).Rates of growth, carcass composition and tissue repartitioning were measured in large (L) and very large (VL) steers implanted with different growth regulators. All cattle were individually fed a whole shelled corn diet. Overall average daily gain (ADG) and feed efficiency (F/G) were better for VL vs L. Daily dry matter intake (DDMI) and days on feed were also greater for VL vs L. Hot carcass weight (HCWT) was greater for VL vs L. Carcass protein and fat (kg) were greater for VL vs L while percentage composition was similar. The VL steers had a larger ribeye area and fat thickness with similar yield grades. Very large cattle had greater numbers of adipocyte cells with similar cell diameters to L steers. Total retail product yield was 43 kg greater and percentage retail product yield was similar for VL vs L. Daily rates of EB gain (DEBG) were greater for VL vs L for all phases of growth. Rates of daily protein gain (DPG) were similar for VL and L for the growing phase and greater in VL for the finishing phase and overall. Average daily gain, F/G and DDMI were greater for implanted steers over N. Implanted steers were fed on the average 13 d less than N. Hot carcass weight was 5% greater for implanted steers over N. Carcass traits reflected the greater leanness of implanted cattle. There was no significant effect of implant treatment on adipocyte physiology. Total retail product yield was greater for all implants. On the average, all implant strategies increased DEBG and DPG and decreased the percentage of fat in daily gain vs the non implanted cattle during all phases of growth. These repartitioning effects were greater at later stages of growth and at faster rates of growth. In both L and VL cattle, implant growth regulators improved growth rate and repartitioned growth from fat to protein, with the magnitude of repartitioning increasing with greater rates of growth and advanced stage of maturity. No major detrimental effect was observed for energetic parameters. Implants reduced the efficiency of ME utilization for growth with small effects on maintenance. Results from this study indicated that very large frame steers and the incorporation of anabolic implant strategies, regardless of the steer size, are viable tools to improve the production of lean beef